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User blog:Samuli.seppanen/Total rebuild
Today, after documenting the stretching process, I inspected the damage on the cheiroballistra caused by the previous dry-fire. One of the strong screws that kept the little ladder beams attached to the rungs had become partially loose, which is probably enough to explain the dry-fire. I suspect the hole for the screw was drilled a bit too large, so that the threads had not sunk deep enough: the other three screws had remained perfectly static. The field-frame on this weakened side had also canted slightly backwards, even though the reason is not entirely clear. The T-clamp that also came off during the dry-fire also came from this side, but it is not clear whether that had contributed to, or was the result of the dry-fire. In any case the other field-frame was in perfect 90 degree angle to the case with no damage whatsover. On top of the little ladder damage I noticed that the straight field-frame bars were very slightly bent, probably about 1mm outwards on both field-frames. While this seems very little, any further increase in tension would give the torsion spring cord increased leverage over the field-frame bar. In other words, the capability of the field-frame bar to stay straight under pressure would start deteriorating rapidly as the bar bent more and more under increased tension. So my hope for increasing the pretension from 320hz to 350hz is a no-go for now. Not surprisingly the curved field-frame bars are still entirely straight, because they are much wider at the center. Anyways, now I'm probably at the right point to do a total rebuild of the cheiroballistra. Here's the list of issues that need to be fixed/solved. These are primarily for my own reference, obviously: # Little arch #* Too wide by about 6cm #* The ends are a bit too weak and thus bend easily #* The curved shape of the forked end makes inserting the ends into pi-brackets more difficult than it needs to be. The shape of the fork also does not match that in the Orsova artifact. #* The curve in the middle is too pronounced and may bend inwards as the draw weight of the weapon is increased. The curve's dimensions are also larger than those given in the manuscript. # Little ladder #* Too wide by about 6cm #* The beams are probably slightly too thin at 4mm. I have partially ready beams that are 5mm thick, which are probably strong enough with 20-56% increase in strength, depending on the direction of the stress. #* A separate rectangular tenon riveted to the beams probably makes more sense than the current approach of forging the ends of the beams into the tenons. #* Two rivets going through each rung would make the entire package much more rigid than one screw on each side, even though the latter approach has worked surprisingly well. Note that many reconstructions have used two rivets, even though it technically violates the description in the manuscript. #* With two rivets it will be possible to replace the birch crosspiece and rungs with ones made from lighter wood that weighs slightly less. # Field-frame bars #* The straight bars are almost certainly too weak to withstand spring cords tuned to 350hz; replacing the 8mm unhardened spring steel bars with 10mm ones will make them about 56% stronger, which should be enough to withstand 350hz springs, which should produce the amount of bolt energy I'm aiming at, i.e. 150-200 joules. That said, an asymmetrical bar that is thicker at the middle would have better weight-strength ratio. #* The lower pi-brackets should be placed slightly higher towards the center of the field-frame bars. This would move the bowstring downwards, thus reducing the possibility of dry-fires #* The ~2.5mm thick pi-brackets have endured surprisingly well, but making them from thicker steel would help. Something like 4mm should be good enough. # Field-frame rings #* These are slightly too thin and have bent in the past. Currently they are only ~5mm thick, so I'll replace them with 7-8mm rings. #* Their profile will need to be changed to the standard Orsova/Lyon style, i.e. pointed ellipse. This would allow the field-frame bars to fit comfortably outside the washer rim. This profile change will require them to be from steel plate instead of being forged from round stock. The hole for the washer core can probably be punched and finished with a file. # The washers #* The washers are probably good enough for 350hz pretensioning. The rims of some of them are are slightly canted upwards (i.e. in the air), but that does not seem to cause much harm. Hopefully I can avoid making new washers, as that's really nasty business. # T-clamps #* These need to be attached better to the case and the little ladder beams. Making small notches to the bottom side of the little ladder beams for the ends of the folded T-clamps would help, as would a small strip of wood that would hold the T-clamps against the case. # Slider #* The end of the slider should be redone from stronger, more split-resistance wood: currently it's birch, but elm would be better #* The Xanten-style dovetail would be easier to make and more robust #** The slider would hve rectangular cross-section with narrow strips of wood attached to the bottom part #** The case would have similar strips of wood at the top part # The case #* The case is fairly heavy because of its large dimensions and the fact that it's made from birch, a medium-weight hardwood. I will replace birch with pine, which will reduce weight of the case by 10-20%. This will compensate for the increased weight in the metal parts. # The arms #* The cones are 3 dactyls too long. This is (now) obvious because they will not be able to rotate without touching the case once the length of the little ladder and little arch has been fixed. #* The spring cord crushes the arm signficantly right next to the hoop at the belly of the arm. This makes the arm bend more at this critical section. This section needs to be either plated with a stronger material, or made thicker in the first place. # Bowstring #* Once the correct length little ladder and little arch have been prepared, a new bowstring has to made. There is no point in making one before that. Anyways, that's plenty of stuff to do, and definitely enough to not bother with trying to patch existing parts. Better start from scratch and fix everything (hopefully) for good. Before that, though, there's plenty of CAD drawing and planning ahead. Category:Blog posts Category:Backup